As crude oil is depleted from an underground formation, pressure in the formation decreases to the point that oil must be pumped to the surface. One of several methods for removing crude oil from an underground formation employs a pump-jack located on the surface. The pump-jack is connected via a sucker rod string to a downhole pump at the bottom of the producing oil well. The sucker rod string comprises many sucker rods, each rod connected end-to-end to another rod by a coupling. The entire rod string extends down into a tubing string that is commonly contained within a well casing. The exterior well casing and internal tubing string are permanently installed after drilling the well. The tubing string serves as a conduit for the fluid produced, and the driving force for this production is transmitted to the downhole pump via the sucker rod string positioned within the interior of the tubing string.
The sucker rod string commonly reciprocates inside the tubing string as a result of the upward and downward motion of the pump-jack to which the rod string is fastened. Cyclical upward and downward motion of the pump-jack is thus communicated to a downhole pump located at the lower end of the tubing string. In response, the pump forces the produced fluids collected at the bottom of the well up the tubing string to the surface. In other applications, a progressing cavity (PC) pump is used at the bottom of the well, and in these applications power to the pump is transmitted via a rotating sucker rod string.
The production fluid in the tubing string typically acts as a lubricant for the sucker rod string. Lubrication is derived from the fluid because it is commonly a mixture which includes crude oil, along with water and natural gas. Typically also included in the production fluids are dissolved and undissolved salts, gases and other formation minerals, such as sand. The recovered crude oil is commonly stored in a tank near the well until it is removed for refining. Natural gas is removed in a pipeline. Water is usually reinjected into the production formation or in a disposal well in another formation close to the production formation.
Due to deflections of both the tubing and the rod string, contact may occur between these components. Even though the lubricating bath of the production fluid is present in the tubing, wear is incurred on the rod string and tubing when contact is made. The rod couplings typically have the largest outer diameter of the various components of the rod string and therefore incur, and cause, the most wear. Produced fluids that flow in the rod and tubing annulus also cause wear in the form of abrasion and corrosion. Through time, all these wear factors may lead to parting of the rod string or to the development of holes in the tubing.
When a hole develops in the tubing, pressure is lost inside the tubing. Production will then be pumped into the annulus between the tubing and the casing rather than to the surface for collection and storage. When a sucker rod separates, when a rod coupling breaks, or when holes are created in the tubing, the sucker rods and/or tubing must be pulled from the well and inspected in detail for the extent and nature of the damage. Damaged rods and tubing must be replaced. The resultant down-time as well as the workovers are a great expense to the well owners. Therefore, methods and apparatus for reducing or eliminating costs associated with lost production of hydrocarbons, equipment replacements and workovers are of great benefit to the well owners.
A well known method of preventing wear to the rods and tubing is the use of rod guides, also known as centralizers and paraffin scrapers. In cases involving a reciprocating rod string, paraffin scrapers may also serve as centralizers to reduce wear, in addition to their implied purpose of removing paraffin from the walls of the tubing. Rod guides have a greater outer diameter than other parts of the rod string. As such, the guides are sacrificial and protective. Rod guides retard rod and tubing wear by incurring most of the wear that does occur.
On the average, six rod guides are normally attached at various locations on each sucker rod in the rod string, but as many as ten or more locations per rod or as few as one location per rod may be used. As such, the guides act as a sacrificial and protective buffer between the rod string and the tubing. Wear occurs to the guide as it protects the rod string and the tubing and results in a reduction of the protective thickness of the guide over time.
The wearing effects suffered by the rod guides will eventually cause the guides to have an outer diameter which will approach and become similar to the diameter of the couplings or parts of the rod string larger than the shank or body of the sucker rod. When this happens, the guide will no longer buffer the contact between the rod string and the tubing. The rod guides must then be replaced.
The general state of the art may be gathered by reference to a Rod Guide/Centralizer/Scraper Catalog published in 1997 by Flow Control Equipment (FCE) Inc. This catalog discusses rod guide material selection, paraffin scrapers; classic rod guide designs such as the standard and slant blade; high performance designs such as the NETB, Stealth and Double Plus; rotating rod guides for PC pumps such as the Spin-Thru and the PC Plus; and field installed guides (FIG's) such as the Lotus twist-on, NEPG, Lotus Rubber and Guardian polyguides. Also relevant to the general state of the art are patents to rod rotators and stabilizing bars.
Many of the design considerations applicable to any rod guide for either rotating or reciprocating sucker rod strings are discussed in a 1993 publication by Charles Hart entitled "Development of Rod Guides for Progressing Cavity (PC) Pumps", a 1995 publication by Randall G. Ray entitled "Determination of Rod Guide Erodible Wear Volume," and a 1993 publication by Milton Hoff entitled "Hydraulic Drag Forces on Rod Strings." The general concept of erodible wear volume EWV and specific formulations as "gross" and "net" EWV are used herein in accordance with the use in these publications. In particular, the portion of a rod guide between the largest outer diameter on the rod string (typically the coupling diameter) and the inner diameter of the tubing string is the volume of the guide which can prevent damaging metal-to-metal contact. This protective volume of the rod guide is referred to as EWV. EWV is an important indicator of rod guide performance. The amount of the rod guide outside the outer diameter of the sucker rod couplings is in general referred to a Gross Erodible Wear Volume or Gross EWV. A more refined concept, which is known as Net Erodible Wear Volume, is that amount of the rod guide material that will erode before the sucker rod coupling contacts the tubing. Net EWV is always less than Gross EWV in conventional rod guide designs when the rod string is reciprocated to drive the downhole pump. Even a reciprocating rod string should be slowly rotated during reciprocation to maximize the useful life of the rod guides. An underlying assumption of both of these EWV definitions is that the rod string is continuously rotated and that the rod guides wear evenly. Also, both definitions are based on the assumption that the rod string is in tension and not in compression. In some rod guide designs, the Gross and Net EWV may be almost the same but as they approach equality, then fluid bypass area decreases and the flow resistance or drag around the guide increases to unacceptable levels. It is the primary objective of the invention presented herein to generate more efficient rod guide designs which have Gross EWV approximating Net EWV without sacrificing the necessary bypass area and geometry necessary to achieve desired levels of flow resistance or drag.
For clarity and ease of discussion, a rod guide may be considered to have a radially inner non-erodible zone and a radially outward erodible zone. The boundary line between the two zones, namely the erodible and the non-erodible zones, will be considered to be the projected circumference of the largest outer dimensions of any component anticipated to be on the rod string in the operative region where the respective rod guide is located, which typically will be the rod couplings above and below the respective rod guide. "Operative region" means that section of the rod string close enough to the rod guide so that it may be expected that the rod guide will furnish some protection to the rod and its couplings. It is meant to exclude for definitional purposes couplings or other rod string elements which may be several rod lengths away from the rod guide and which would have no effect on the function or performance of the rod guide, and thus no effect on the guide dimensions at issue. As used herein, the terms "by-pass" and "flow through" are intended to be synonymous and interchangeable.
U.S. Pat. Nos. 586,001 and 1,600,577 are directed to a cleaner for oil well tubing and a paraffin scraper, respectively. Both disclosures have a gross similarity to some of the embodiments of the present invention but differ in intent, function, material and design. The same may also be said of U.S. Pat. No. 2,153,787, which is directed to the shrink fitting of a guard by extraction of a plasticizer. A flexible guide is taught in U.S. Pat. No. 2,651,199. A method of on-site molding of scrapers is disclosed in U.S. Pat. No. 3,251,919.
U.S. Pat. Nos. 2,863,704 and 4,997,039 disclose a combination rod guide and sand purging device. Several of the embodiments referred to in the materials cited above are disclosed in U.S. Pat. Nos. 4,088,185, 5,115,863 and 5,277,254. Recently disclosed variations of a rod guide are found in U.S. Pat. Nos. 5,358,041 and 5,492,174.
None of the above references are directed to the concept of the present invention as set forth and described below. The present invention overcomes the deficiencies of the prior art and achieves its objectives by maximizing EWV while providing adequate flow through paths in and around the guide to both prevent excessive hydraulic drag during movement of the guide with respect to the produced fluid and avoid the creation of an excessive pressure drop as the guide passes through the produced fluid during the downward motion of the sucker rod string.